To understand knots, get a really long rope and a boat

The sailor in the middle is holding the log line, a line with a small board on its end and knots tied into the rope at regular intervals. The navigator, left, measures time with an hourglass, and the man letting the line run as the ship moved through water would “call the knots” as they passed through his hand.

The sailor in the middle is holding the log line, a line with a small board on its end and knots tied into the rope at regular intervals. The navigator, left, measures time with an hourglass, and the man letting the line run as the ship moved through water would “call the knots” as they passed through his hand.

Q: On land we measure wind speed in miles per hour. On sea we measure it in knots. Why the difference?

Stephen Krause, of Collinsville

A: Even Tevye, that poor dairyman in “Fiddler on the Roof” could answer this question: It’s tradition.

You see, today’s computer app-addicted kids might be surprised to learn that centuries ago sailors couldn’t rely on GPS satellites or even speedometers to calculate speed. Yet when they were running low on food or facing incoming storms, they needed to know how fast they could reach a safe haven for supplies or shelter in time. But how could they possibly do this?

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“They used materials they had on hand,” explains Camila Caballero, a research assistant at Massachusetts Institute of Technology’s School of Engineering. “A wedge-shaped piece of wood, a small glass timer — and a really long rope.”

But not just any old rope. Based on the length of the nautical mile, knots were tied at intervals of 14.4 meters (15.75 yards). One end of the rope was then tied to the ship’s stern while the other was attached to the wooden board, which was then dropped into the water like an anchor.

“As one sailor watched the sand empty through the 30-second glass, his shipmate held the line as it played out behind the ship and counted the knots as they passed between his fingers,” Caballero said.

Dividing 14.4 meters by 30 seconds told them that each knot equaled 1.15 miles — or one nautical mile — per hour. (A nautical mile is longer than land miles because it was defined as one-60th of a degree of latitude at 48 degrees latitude — or about 6,080 feet.) By adding up the total numbers of knots that unspooled, the sailors were able to calculate the ship’s speed.

The average of frequent measurements taken throughout the day proved to be a highly accurate estimate of how fast a ship was moving, Caballero said. Now, ships use ultrasonic sensors and Doppler measurements to determine speed, and the 30-second time segment has been replaced by 28. But the instrument for measuring a vessel’s speed is still called a log, and both sea and aeronautical distances are still measured in nautical miles.

“Maps used at sea and in the air are based on the earth’s circumference,” Caballero said. “Their scale varies with latitude, and the nautical mile, about 500 feet longer than the land mile, reconciles those differences.”

As a result, speed at sea still is based on the knot, which is equal to one nautical mile. To help landlubbers, however, meteorologists conveniently convert hurricane speeds from knots to mph by multiplying by 1.15. But if you want to have your friends saying, “Well, blow me down!,” you can show them how smart you are by converting distances and wind speeds from mph to knots by multiplying by .87.

▪ Driving them batty:My recent trivia question about the fastest flying creature on Earth prompted several calls questioning my facts, but as usual you can trust the Answer Man (in this case, at least).

A new study found that the Brazilian free-tailed bat can shoot through the night skies at roughly 100 mph, easily eclipsing the previous mark of about 68 mph set by the common swift. But this bat weighs only 11-12 grams (about half an ounce), which had people wondering how its speed was determined. In other words, how could anything that light possibly overcome the wind resistance to go that fast?

Rest assured, the scientists have hard evidence to back their finding. They glued a radio transmitter weighing just half a gram to the bats’ backs. (Don’t worry, the adhesive lost its stick-um after a few days, and the tiny instrument fell off.) Its regular beeping signal was localized using a mobile receiver installed on a small airplane. The scientists also evaluated the data recorded by the closest weather station and noted the wind conditions at the time of the studied flights.

“External factors like landscape and tailwinds cannot explain these results, as they had no impact on the maximum speeds,” said Dina Dechmann, of the Max Planck Institute for Ornithology in Stamburg, Germany.

Dig this twofer: Another trivia question about the National Mining Hall of Fame brought this invitation from Robert Johnson, of Johnson Consulting in Collinsville. While en route to the HOF in Leadville, Colo., think about stopping at the Kansas Underground Salt Museum in Hutchinson. Now known as Strataca, the museum is built within one of the world’s largest deposits of rock salt and offers the chance to go 650 feet underground. Open since 2007, the mine has an underground train and even offers a Murder in the Mine dinner theater. See underkansas.org for details.

Brick solid: My recent column on the Richards Brick Co. in Edwardsville had Phillip Flach, of Belleville, reminiscing about his great-grandfather John Gansmann, who was part-owner of the Gansmann & Mueller Brick Co. at North Illinois Street and the Louisville and Nashville tracks in Swansea.

“I suppose the business closed with the Depression,” Flach wrote. “It made a lot of the bricks paving Belleville streets, as I recall the family lore. His wedding present to a daughter (my grandmother, Frances Flach) was the brick house at 1105 N. Church St.”

Today’s trivia

What film, which was nominated as worst movie of the year in 1993, was turned into a Broadway show — and earned a Tony nomination for best musical of the year in 2012? (Same title for both.)

Answer to Friday’s trivia: In the 1930s, Thomas Dam was trying to keep his poor family in Denmark afloat by selling his carvings made from wood. One of his creations was a large troll doll, which became a popular item at Danish fairs. By the 1950s, he was making smaller versions out of rubber topped by wigs sewn together from small pieces of sheepskin. Soon, he had to build a Good Luck Troll Dpolls factory as international demand exploded for his homely creation. Dam died in 1986 at age 76, but his Dam Things live on at www.damworld.dk. Also, check out the new Toys of the ’50s, ’60s & ’70 free exhibit at the St. Louis History Museum to see some trolls. Plus, DreamWorks Animation in 2011 acquired the film rights to the Good Luck Troll toys from Dam Things to produce the animated movie, “Trolls,” which is in theaters now.